Enzymatic resolution of prochiral and racemic compounds has become a valuable and widespread technique, (C. H. Wong, G. M. Whitesides, Enzymes in Synthetic Organic Chemistry, 1994, Pergamon Press, New York). There are a variety of methods available for the resolution of racemic or diasteromeric mixtures of molecules which include esterification, de-esterification, acylation, de-acylation, hydrolysis and reduction. Either the desired or undesired isomer may undergo the chemical change as long as the reaction is sufficiently selective for that isomer. Typically, a preparation of a chiral molecule may proceed by the conversion of one of the enantiomers or diasteriomers of a mixture with the correct enyzme. However, simple enzymatic resolutions are restricted to providing a maximum 50% yield of the optically pure product based on racemic starting material. Either the wrong isomer must now be discarded or it is racemized back to a mixture similar to the original mixture. The racemized mixture may now be resubmitted to the enyzmatic resolution conditions as before. Assuming the yield of conversion and recovery to be 100%, the original mixture could be converted to the desired chiral isomer in a time consuming reiterative fashion, each step converting 50% of the racemic starting material of that step. The overall yield would asymptotically approach 100% if the process was infinitely continued. However, in practice, this is rarely possible due to the tedious repeated operations and losses during isolation.
Isoxazolines are important components in pharmaceutically active and agriculturally active compounds. Published examples of active isoxazoline compounds or processes for making isoxazoline compounds include, but are not limited to, U.S. Pat. No. 4,970,297 (transglutaminase inhibitors), U.S. Pat. No. 5,489,562 (herbicides), U.S. Pat. No. 4,256,898 (antitumor and antimicrobials), U.S. Pat. No. 4,933,464, U.S. Pat. No. 4,952,700, PCT International Publication WO 95/14681 (antiinflammatory agents), PCT International Publication WO 95/14680 (antiinflammatory agents), and PCT International Publication WO 95/24398 (inhibitors of TNF release).
Compound (X), is a useful antagonist of the platelet glycoprotein IIb/IIIa fibrinogen receptor complex. ##STR1## As such compound (X) is useful in the inhibition of platelet aggregation, as a thrombolytic, and/or the treatment of thrombolytic disorders. The preparation of compound (X) has been disclosed in the following references: U.S. Pat. No. 5,446,056, herein incorporated by reference, PCT International Publication WO 95/14683, PCT International Publication WO 96/38426 and Zhang et al. Tetrahedron Lett. 1996, 37, 4455-4458. These documents describe the key role played by compound (R)-(IIa) as an intermediate in the total synthesis of compound (X). ##STR2##
The current synthetic route to compound (R)-(IIa) consists of an enzymatic resolution of isoxazoline isobutyl oxoesters. The unhydrolyzed ester is then racemized in a second discrete step and resubmitted to hydrolysis conditions. Several repetitions of these two independent reactions eventually yield high conversions of compound (R)-(IIa) but will require an inconveniently large number of isolations and individual reactions (Scheme A). ##STR3##
The instant invention achieves the above two step transformations in a single step. Hydrolysis and racemization occur simultaneously by the use of a thioester in place of the oxoester and modification of the reaction conditions, permitting the isolation of compound (R)-(IIa) in one step in equivalent purity and higher yield as compared to the original preparation.
D. G. Drueckhammer, et al., J. Am. Chem. Soc. 1995, 117, 9093-9094, have achieved simultaneous hydrolysis and racemization where the chiral center is the alpha carbon to a thioester. In their work, the alpha proton's acidity is enhanced by the presence of an alpha phenylthio group in addition to a thioester. This enables racemization and hydrolysis to occur at similar rates to produce a &gt;99% conversion to the desired product (R)-2, (Scheme B). Drueckhammer, et al. acknowledge that a thioester of a substrate having only saturated alkyl substituents on the alpha carbon would not be sufficiently acidic to permit racemization under the conditions used in their work. ##STR4##
The instant invention concerns the beta carbon whose proton acidity is less then the alpha carbon proton. Therefore, it is hypothesized that racemization at the beta carbon, enhanced by the thioester, proceeds under basic pH by a mechanism of isoxazoline ring opening at the carbon oxygen bond. ##STR5##
The invention combines two processes: racemization of a chiral center at the beta carbon to the thioester and the enzymatic hydrolysis of the thioester. This combination into a process of dynamic resolution constitutes the value of the invention. This invention discloses a method for the resolution of carboxylic acids substituted at the beta position with an isoxazoline ring. More preferably, this invention reveals a superior method to prepare compound (R)-(IIa), an important intermediate within the synthetic sequence to prepare compound (X).
The possibility to conduct enzymatic resolutions of racemic molecules and obtain a conversion in greater than 51% yield to optically pure product without the necessity of conducting a separate racemization step is of great potential value. This process saves time and money as it minimizes the number of reactions that must be run to obtain chiral product. This process decreases preparation times, reduce the cost of the product by minimizing the number of manipulations and reagents/solvents used, and increases the yield.